Conventionally, a speed light device (flash unit) using a xenon tube has been used as an auxiliary light for a still image in the case of a silver salt camera, a video camera and an electronic still camera. The speed light device charges a main capacitor with a high voltage of 300 V or so, and has this charging energy discharged via the xenon tube to convert it to emission energy and illuminate an object. Therefore, a high-voltage portion is laid out on a compact body, and so there are many layout restrictions in terms of substrate mounting, such as a creeping distance problem including pattern spacing and placement of a large main capacitor.
As opposed to this, a white LED can emit light by applying a voltage of a few V. Therefore, it has no problem of a creeping distance from another component in the case of handling the high voltage. If the voltage and performance of a battery are high, the white LED can be driven directly from the battery so that it no longer requires the main capacitor taking a large space, which is a merit in view of miniaturization.
The white light-emitting diode (hereafter, white LED) is now used as the auxiliary light for a camera cell-phone, a camera for medical use and so on.
However, the white LED has a luminance remarkably lower than the xenon tube used for the speed light device. For this reason, it has been used, due to its limited shooting distance, for a macro speed light device used at a short distance and the camera cell-phone for photographing in a handheld range.
A light volume of the speed light device is indicated by a guide number (hereafter, GNo.), and is decided by ISO sensitivity, shutter second time (also referred to as shutter speed, same hereafter) and a light volume (cdsec) from a light source. For instance, if the white LED of 10 cd is energized for 1/30 second and the ISO sensitivity is 100, GNo. is 0.41. GNo. of a general built-in speed light device is 6 to 14 or so, where it requires the light volume of about 215 to 1200 times in order to have this light volume.
However, current low-noise techniques have advanced, and the ISO sensitivity can also be set high. Recently, it is reaching a level where there is no problem in terms of photographing even if the ISO sensitivity is increased to ISO 800.
Luminous efficiency of the white LED itself is also improved at a rapid rate, and it is expected as the auxiliary light of the image capturing apparatus. The efficiency of the current general white LED of a high-luminance type has reached approximately 40 (Im/W) and will reportedly reach a fluorescent level (80 to 100 Im/W) in near future. The white LEDs of a power-type of 1 W or more are currently at 20 to 251 Im/W, and the efficiency thereof is also being improved at a rapid rate likewise. Therefore, in the case where the ISO sensitivity becomes 8 times higher and the efficiency of the white LED becomes 3 times higher, a necessary light volume will be reached by 9 to 50 white LEDs and so it is only one step away from practical use. The white LEDs have also reduced cost.
Given the present circumstances, however, the luminance of the white LED is lower than that of the xenon tube and so multiple pieces thereof must be used, which is a significant problem in terms of the cost and space.
Therefore, it is necessary to reduce the required quantity to the minimum quantity. If the shutter second time on photographing is extended to obtain the light volume for the reason of the insufficient luminance, there occurs a problem of hand shake even though the guide number becomes larger.
As for the problem of the hand shake, there is a presented technique for the electronic still camera as an effective correction technique, wherein multiple images are continuously shot in the shutter second time enough to cause no hand shake and are composed while being aligned in a process after the photographing so as to obtain the images with no hand shake (Japanese Patent Laid-Open No. 2002-064743 for instance).
For instance, according to Japanese Patent No. 3110797 proposed by the inventors hereof, there is a disclosed technique wherein sequentially captured images are composed after performing coordinate transformation on multiple screens correspondingly to displacements over time.
According to Japanese Patent Laid-Open No. 9-261526 for instance, there is a disclosed technique wherein multiple images are shot likewise and are composed while correcting the displacements so as to obtain the images with no displacement.
According to Japanese Patent Laid-Open No. 2000-299813, there is a disclosed technique wherein continuously shot image data is recorded as motion JPEG data together with hand shake data and is reproduced while correcting the displacements according to the recorded hand shake data on reproduction.
However, even the techniques disclosed in the above documents have not resolved the issue of correcting the hand shake and reducing the used quantity of the white LED generating auxiliary light to reduce the cost.
To be more specific, Japanese Patent No. 3110797 discloses an example of applying it to the video camera. However, it does not disclose a technique for performing a hand shake correction and a synthesis in post-processing, and so it is not related to a method of efficiently using the auxiliary light.
Japanese Patent Laid-Open No. 9-261526 discloses an application to the electronic still camera. However, it only discloses an embodiment performed in the camera as to the hand shake correction and synthesis as with Japanese Patent No. 3110797. Therefore, it does not propose the method of using the auxiliary light.
Furthermore, according to Japanese Patent Laid-Open No. 2000-299813, the continuously shot multiple images are recorded as a motion picture of Motion JPEG so that the multiple images are associated. However, it is intended to reproduce them while correcting the displacements on reproduction. And it discloses no means for obtaining one image with no hand shake by means of image composition. It is not related to the use of the auxiliary light.
Japanese Patent Laid-Open No. 2002-064743 discloses a method for generating a bright image with no hand shake by composing multiple images. However, it does not describe emitting light from the white LED by using the white LED as illumination unit.
The present invention has been made in view of the problems, and an object thereof is to realize an image capturing technique capable of obtaining an image of a correct exposure with no hand shake by using no illumination of which light volume and power consumption are high such as the speed light device.
Another object is, on using the white LED as the auxiliary light, to reduce the used quantity of the white LED in order to solve the problems in terms of the cost and space. For this object, multiple images are shot with the auxiliary light in the shutter second time enough to cause no hand shake and are composed while correcting a hand shake amount so as to reduce the used quantity of the white LED and obtain the images with no hand shake. It is not always necessary to perform the process of the hand shake correction and synthesis in the electronic still camera. It is also possible to only shoot the multiple images in the electronic still camera and perform the hand shake correction and synthesis by capturing them on a personal computer (hereafter, a PC).
When post-processing the images on the PC, it is possible, with many of single-lens reflex electronic still cameras and high-class compact electronic still cameras, to store raw data images obtained from an image capturing element as-is (generally called raw images). The raw images cannot be seen as-is as the images. However, they can be converted to JPEG images while performing color temperature correction, tone correction and exposure compensation (referred to as a development process because of a silver salt image) on the PC. Such post-processing on the PC is generally performed.
Furthermore, it is necessary to reduce the time from image capturing to image recording in the electronic still camera due to demands for the increased number of pixels of the image capturing element and improvement in continuous shooting performance. Thus, there is a merit of reducing a load on the electronic still camera side by performing the hand shake correction and synthesis in the post-processing. The present invention also reduces the used quantity of the white LED and obtains the images of a correct exposure by using the white LED as the auxiliary light and shooting multiple images of a low luminance requiring a low light volume to compose the obtained images in the post-processing.